In color diffusion transfer photography, the so-called redox compound (hereinafter, "DRR compound") which releases a diffusible dye has been used as the color image reproduction material (hereinafter "dye material"). Photographic Science and Engineering, 20, No. 4 July/August, 1976, P. 155-160 and T. H. James et al.: The Theory of The Photographic Process, The MacMillan Publishing Co., Inc., Third Edition, P. 300 and Fourth Edition (1977), P. 322 state that in order to cause a DRR compound to release a diffusible dye in an imagewise pattern corresponding to the exposure of the silver halide, it is advantageous to employ a derivative of 1-phenyl-3-pyrazolidone (pyrazolidinone), particularly 1-p-methylphenyl-4-methyl-4-hydroxymethyl-3-pyrazolidone, as an electron transfer agent. The rationale of using such an electron transfer agent is that not only a high level of development activity can be assured but the compound itself is readily soluble and stable in alkaline processing compositions.
U.S. Pat. No. 4,209,580 (corresponding to Japanese Patent Application (OPI) No. 52055/80 and European Patent No. 0009837B1) also teaches that 1-p-methyl-m-methylphenyl-4-methyl-4-hydroxymethyl-3-pyrazolidone and 1-m,m'-dimethylphenyl-4-methyl-4-hydroxymethyl-3-pyrazolidone as well as precursors thereof can be used as electron transfer agents with the same advantageous as those referred to above.
However, irrespective of whether it is a "monosheet" type film unit comprising an image receiving element and a light sensitive element disposed in superimposition on a single support or a film unit such that an image receiving element and a light sensitive element are disposed on independent supports with a processing composition being spread between the two elements, the conventional film unit has the disadvantage that as the image receiving element carrying a diffusion transfer image is separated from the light sensitive element and the processing composition is exposed to the atmosphere, the 3-pyrazolidone (pyrazolidinone) type electron transfer agent contained therein gives rise to intense pink stains (as well as yellow stains depending on the compounds). This seriously detracts from the aesthetic quality of the transfer image.